1. Field of the Invention
The present invention relates to a diffraction grating with alternately-arranged different regions having different polarization dependencies, a semiconductor optical device with this diffraction grating, such as a distributed feedback semiconductor laser, a distributed Bragg reflector semiconductor laser and an optical amplifier whose polarization dependency of its amplification factor is reduced. The present invention also relates to a light transmitter, a light receiver and an optical communication system using the semiconductor optical device of this invention.
2. Related Background Art
Conventionally, as an oscillation polarization-mode switchable semiconductor laser whose polarization mode of output light can be changed between a transverse electric (TE) mode and a transverse magnetic (TM) mode depending on its stimulated condition, the following device has been developed and proposed. This is disclosed in Japanese Laid-Open Patent Application No. 2-159781 and is a three-electrode, buried-type distributed feedback (DFB) semiconductor laser which has an active layer of bulk crystal and a diffraction grating with a .lambda./4 shift section, for example. In this semiconductor laser, since the active layer is composed of the bulk crystal, amplification factors for light in the TE mode and light in the TM mode are substantially equal to each other. In this structure, thresholds for light in the TE mode and light in the TM mode are varied by controlling the amount of current injected into a region with the .lambda./4 shift section, and light in a polarization mode having the lower threshold value is oscillated and output therefrom.
Thus, in order that the polarization mode of the output light is varied depending on the stimulated condition, the amount of carriers injected into the .lambda./4 phase shift region, in which the polarization-mode dependency of the amplification factor and loss of the active layer are small, is changed, and hence the radiative wavelength, at which the round-trip phase matching condition is satisfied for each of the TE mode and the TM mode, is changed. Accordingly, the threshold values for the TE mode and the TM mode are changed and the relationship of magnitude therebetween is alternately reversed.
Further, Japanese Laid-Open Patent Application No. 5-129719 discloses a distributed feedback semiconductor laser in which a diffraction grating is formed using a strained superlattice. In this structure, the strained superlattice layer having a lattice constant different from its substrate is periodically formed with a pitch of the diffraction grating and periodical portions of a quantum well active layer formed on the strained superlattice layer are subjected to strain of the periodically-formed strained superlattice layer. Thus, the quantum well active layer is composed of periodical portions subjected to the strain and the other portions without any strain to substantially form the diffraction grating, so that a gain-coupling type DFB semiconductor laser is built.
The device disclosed in Japanese Laid-Open Patent Application No. 2-159781, however, has a drawback that its operation current is large since the active layer is composed of the bulk crystal.
Further, in the device disclosed in Japanese Laid-Open Patent Application No. 5-129719, although the operation current is lowered since the active layer has the quantum well structure, it is difficult to regulate the relationship between gains for the TE mode and the TM mode (for example, both are made equal, or one is larger than the other) because the quantum well active layer undergoes the strain only indirectly. Hence, a stable polarization-mode modulation is hard to attain therein.